1. Field of the invention
The present invention relates to methods for accurately controlling a medium displacement in an inkjet printer, inkjet printers having a control unit that is configured to determine a relation between an actuation signal and a medium displacement, and methods for determining a relation between an actuation signal and a medium displacement in an inkjet printer.
2. Description of the Related Art
Among the various types of inkjet printers, a scanning-type inkjet printer is known, wherein a recording medium is transported in a specified, transport direction and a carriage, comprising multi-nozzle printheads for applying variously colored inks, is reciprocating in a scanning direction perpendicular to the transport direction in order to print swaths of ink dots, thereby generating an image on the recording medium. During a scanning movement of the carriage across the recording medium, the medium position is fixed. The advancement of the recording medium is performed stepwise at the time the carriage reverses its movement. The accuracy of a recording medium advancement, also known as a paper step, is known to be important, because contiguous swaths applied by the printheads are to complement each other. An inaccurate paperstep would cause a light or dark border line or area between the swaths.
A drive mechanism for achieving accurate papersteps is known e.g. from European Patent 1782960 B1. This mechanism comprises a feed roller having the recording medium pinched onto its surface. Thus, the surface movement of the roller is transferred to the medium. The roller is driven by a drive motor with an angular encoder, or an angle encoding device, on its axis and a slip free transmission that provides a high transmission ratio. A suitable transmission is, amongst others, a worm/wormwheel gear, a gearbox or a toothbelt, possibly multi stage. This has the advantage that a small advance increment of the roller and the medium corresponds to a relatively large angular increment of the motor axis, enabling a high control accuracy of the medium transport. A further advantage is that a full revolution of the motor and the intermediate gear corresponds to an applicable basic stepsize of roller and medium combination. This enables the use of papersteps that correspond to an integer number of basic stepsizes, equivalent to an integer number of full revolutions of the motor axis and the intermediate gear. This helps to achieve a high accuracy in a similar way as is described in U.S. Pat. No. 5,529,414.
In principle, a linear relation exists between an actuation signal, causing the drive motor to rotate to a corresponding angular position of the motor axis, and a displacement of a recording medium, or paper. However, due to eccentricities, unroundness and dimensional errors of the roller, the motor axis, and the transmission, regular, repeating deviations from this linear relation occur. These deviations, or errors, as a function of the actuation signal, or a corresponding circumferential position of the roller, have different frequencies due to the different rotation velocities of the components. A smallest repeating deviation frequency may be identified, associated with the roller and its transmission. This is often the roller frequency, but may also be, for example, the least common multiple of the roller and a belt circumference. By printing a first marker pattern, or reference mark, on the recording medium, displacing the medium over a distance equal to a basic stepsize, printing a second marker pattern besides the first one, usually with a different part of the printhead, and comparing the positions of the two markers, a good estimation of the difference between an actual paper stepsize and a nominal basic stepsize may be obtained. Repeating this procedure enables the determination of this difference in dependence on the angular position of the motor axis and the circumferential position of the roller. A table may be drafted, relating an actuation signal for an integer number of basic stepsizes to a deviation between an actual medium displacement and its nominal value. This table is used by a controlling unit to adjust the actuation signal associated with a required paper step. To appropriately phase the compensation, at least one known configuration or position of the transmission is to be measured, using, for example, a home sensor on the roller. Frequencies which are associated with rotating components other than the roller, may be ignored, due to the fact that these rotating components make full revolutions only. Thus, a full cycle of the higher frequencies is completed.
The method sketched above limits the use of the medium transport mechanism to an integer number of basic stepizes only. This may not suffice to design different print strategies. For example, a basic stepsize of one eigth of a swathwidth allows the application of a four-pass print strategy by using a stepsize of two basic steps or the application of a two-pass print strategy by using a stepsize of four basic steps, but it is incompatible with a six-pass print strategy applying a stepsize of one sixth of a swathwidth. In that case, the higher frequencies do play a role and can not be ignored. U.S. Pat. No. 7,980,655 provides a method for determining a deviation for these high frequencies. In this method a plurality of markers is printed, with the application of a medium displacement that is smaller than the basic stepsize equivalent to a full rotation of the driving components. Unfortunately, if the basic stepsize is rather small, this method does not provide the required accuracy. This is due to the fact that paperslip causes a form of transient behaviour in the displacement of the medium, that is relatively large when a medium step is small. Furthermore, a small stepsize limits the size of the marker that is used to determine the actual paperstep and a smaller marker results in a less accurate determination.
Therefore, a problem exists for determining an accurate relation between a medium displacement and an actuation signal for a medium displacement system with rotating driving components. An object of the present invention is to provide a method that solves the above-mentioned shortcomings.